Total knee arthroplasty, in which the natural femoral and tibial condylar bearing surfaces are replaced with prosthetic femoral and tibial components, has been practiced for many years with a high rate of success. One total knee prosthetic system that has been used with excellent results is the P.C.A..RTM. Total Knee System sold by the Howmedica Division of Pfizer Hospital Products Group, Inc. (New York, N.Y.). The P.C.A..RTM. system includes, in addition to prosthetic femoral and tibial components, a prosthetic patellar component adapted for fixation to the natural patella, which component contacts the anterior plate portion of the femoral component when the knee joint is fully extended and cooperates with the femoral component during flexion of the prosthetic joint.
Despite the success of existing products such as the P.C.A..RTM. Total Knee System, differentiated prosthetic component designs are continuously sought which would more closely approach in use the anatomical function of the natural knee joint. One aspect of this anatomical function is the maximum achievable range of motion in joint flexion, which is about 150 degrees from full extension to full flexion in the natural human knee. Typically, a range of motion in flexion of only about 110 to 120 degrees is obtained with the use of known total knee prosthetic systems. Roughly half of this gap between the natural and prosthetic ranges of motion can be attributed to the inherent clinical and surgical situation, for example scarring within the knee joint, posterior ligament laxity and loss of the superpatellar pouch. However, the remainder of the gap can, in principle, be closed by appropriate modifications of prosthetic component geometries. Of particular concern is the effect of component geometries on the quadriceps muscle which extends from the femur over the patella (kneecap) to the tibia.
The quadriceps muscle experiences significant tension whenever the knee is not fully extended. This muscle controls extension of the knee (by contracting) but resists flexion. Typically, when conventional prosthetic knee components are used the quadriceps tension at flexion angles above about 90 degrees (from full extension) is substantially greater than in the natural anatomic human knee. Additionally, with the use of some prosthetic systems the quadriceps tension continues increasing with increasing flexion beyond an about 90 degree flexion angle, while in the natural human knee the quadriceps tension remains approximately constant in this high range of flexion. As a result of the excessive (and in some cases continuously increasing) quadriceps muscle tension resisting flexion, the maximum flexion angle that can be realized with the prosthetic knee joint is significantly limited.